What is a 'P' trap

[Cecil]

My washer drain has an S trap.  I need to make some changes in that area, so I want to bring it up to code.  What is the minimum length of the trap arm for a 'P' trap.  Thanks

[Roly]

Is this what your doing ?   If so two times the trap diameter.    Roly

[Cecil]

Yep, that is exactly what I am doing.  Thanks

[Roly]

Note how it is measured.     Roly

[blackhat]

That would be better if there was a vent or AAV on the vertical.

[Snipe Hunter]

The reason we no longer use P-Traps is because they siphon the trap dry, making them useless.

[Stwood_]

The reason we no longer use P-Traps is because they siphon the trap dry, making them useless.

Ahha. Learn something every day.


So our kitchen drain is that way (old way). But I don't see a way to change it, as the waste pipe is right there by the sink outlet. My trap is twisted back around itself

[Roly]

The reason we no longer use P-Traps is because they siphon the trap dry, making them useless.

Do you mean S traps instead of P traps ?   Roly

[Cecil]

The reason we no longer use P-Traps is because they siphon the trap dry, making them useless.

Ahha. Learn something every day.


So our kitchen drain is that way (old way). But I don't see a way to change it, as the waste pipe is right there by the sink outlet. My trap is twisted back around itself

The 'S' trap may allow a siphon, not a 'P' trap, according to what I find (https://www.youtube.com/watch?v=bU8aMK2NmI8).  This happens when a high volume of water is drained.  When a toilet drains, it always empties enough of the trap to break the seal and let the sewage gasses into the house.  However, the toilet also replenishes the bowl, to refill the trap.

In my case, I recently purchased a wood shop with a house attached.  The previous DIY owner made many errors.  A laundry was added, with an 'S' trap and no vent on that end of the house.  I am putting on a roof, so I am taking this opportunity to add a vent and, at the same time, putting in a 'P' trap.  I will need to update the kitchen and bath sink as well, eventually.  In the meantime, any time I pull the plug to drain a sink, I remember to follow up with a little from the tap.

[Snipe Hunter]

Do you mean S traps instead of P traps ?   Roly

Yea oops. Sorry fellas.